In general, an insulated electrical wire having an insulating coating film on a surface of a conductor is produced as follows. Specifically, first, an insulating varnish is applied onto a surface of a conductor (core wire). Subsequently, by using a coating die, the amount of insulating varnish applied onto the surface of the conductor (core wire) is adjusted and the surface of the applied insulating varnish is smoothed. The applied varnish is then baked through a baking furnace to form an insulating coating film on the surface of the conductor. This series of operations (namely, the application of the insulating varnish, the adjustment of the amount of coating, and the baking) is repeatedly performed a plurality of times until the thickness of the insulating coating film reaches a predetermined value.
The coating die has an opening, and the core wire coated with the insulating varnish passes through this opening to remove the excess insulating varnish, thereby adjusting the amount of coating. In the case where an insulating coating film of an insulated electrical wire has an uneven thickness, the electrical insulating property of the insulated electrical wire is determined by a portion where the thickness of the insulating coating film is small. Therefore, the insulating property may be determined to be insufficient. Accordingly, it is important to make the film thickness uniform. For example, as described in Japanese Unexamined Patent Application Publication No. 2008-123759 (PTL 1), a coating die having an opening whose shape in plan view is substantially similar to the cross-sectional shape of a conductor is usually used.
Hitherto, a conductor (round wire) whose cross section has a substantially circular shape has been used as the conductor (core wire) of an insulated electrical wire. However, recently, a conductor (flat conductor) wire whose cross section has a substantially rectangular shape (whose cross section has a substantially flat shape) has been widely used from the standpoint of a high space factor and the possibility of the realization of reduction in the size of various devices. In the case where an insulating coating film of such a flat electrical wire has an uneven thickness, in the state of winding, the electrical wires do not directly contact with each other and unnecessary spaces may be formed, which may result in the decrease in the space factor. In the flat electrical wire, the uniformity of the coating film is important for the above reason. However, it is difficult to make the thickness of the coating film uniform, as compared with the case of a round wire.
Specifically, as illustrated in FIG. 1, in the case of the preparation of a flat electrical wire in which a core wire 1 has a coating film 2 having a uniform thickness, as illustrated in FIG. 2, a die having an opening 3 having a shape similar to the shape of the core wire 1 is used. However, in a flat electrical wire having both a planar portion 1a and a straight down portion 1b orthogonal to the planar portion 1a, while the flat electrical wire passes through the die and is then cured, an insulating varnish easily flows in corners of a flat conductor due to the surface tension of the insulating varnish. Consequently, an insulating coating film whose cross section has a so-called dog bone shape, in which corners 2′a of a coating film 2′ project, is formed, as illustrated in FIG. 3.